This invention relates to improved basic polyether polyols, to a process for their preparation, and to their use for the preparation of polyurethane and polyisocyanurate plastics.
Alkylene oxide adducts of tolylene diamines ("TDA"), in particular of 2,4- and 2,6-TDA and mixtures thereof, and their use for the preparation of polyurethanes has long been known (British Patent 972,772). Polyethers based n o-TDA and their use for the preparation of polyurethanes are also disclosed in German Offenlegungsschrift 2,017,038. Alkylene oxide adducts of aniline-formaldehyde condensation products ("MDA") are described in U.S. Pat. No. 3,499,009. In these polyethers, all amine protons of the aromatic amines react with epoxides.
TDA is manufactured in large quantities as a starting material for the preparation of tolylene diisocyanate ("TDI"). Due to its ready accessibility, crude TDA, which contains small quantities of o-components in addition to the 2,4-and 2,6-isomers, has always been an attractive candidate as starting material for rigid foam polyethers. In the manufacture of TDA, it has become customary to purify crude TDA by distillation, a process in which o-TDA (a mixture of 2,3- and 3,4-TDA with up to 15% by weight of impurities such as diaminocyclohexane, hexahydrotolylenediamine, aniline, toluidine, and the like) is obtained as by-product. This inexpensive by-product is increasingly being used, both in the directly obtained crude form and as a purified form, as starting material for basic polyethers.
Work in this field has intensified following reports (e.g., U.S. Pat. No. 4,209,609, believed to be equivalent to EP 001,800) that rigid polyurethane foams based on TDA polyethers have important advantages over conventional rigid polyurethane foams if the OH number ranges from about 400 to about 630 and if such foams have been prepared by first reacting TDA with approximately 4 moles of ethylene oxide and then with propylene oxide. That is, in addition to the end product having a high OH number and a minimum proportion of ethylene oxide, almost all of the end groups must be secondary. Particular advantages of these products include an extremely low coefficient of thermal conductivity, high flame resistance, and excellent toughness.
Pure propylene oxide polyethers prepared by conventional processes are very highly viscous in the above-mentioned range of OH numbers (e.g., greater than 50,000 mPa.s at 25.degree. C.) and give rise to rigid foams having only ordinary levels of properties. Such ordinary properties are also found for relatively low viscosity pure propylene oxide polyethers, such as those prepared according to U.S. Pat. Nos. 4,391,728 and 4,421,871 in the presence of at least 0.8% alkali metal hydroxide at high temperatures. Pure ethylene oxide polyethers, on the other hand, are too reactive for most applications of rigid foams because of their primary hydroxyl groups.
Suitable polyethers which satisfy the above-mentioned requirements by containing about 4 moles of ethylene oxide per mole of TDI as well as propylene oxide, are described in U.S. Pat. No. 4,209,609. According to this patent, the alkoxylation catalyst is preferably added after the ethylene oxide stage. If, however, one of the above-mentioned amine catalysts (such as triethylamine or pyridine) is used before the addition of the ethylene oxide, highly viscous products containing exclusively tertiary amino groups are obtained. When potassium hydroxide is used directly, very low viscosity products are obtained but the results are not reproducible (see German Offenlegungsschrift 2,017,038, page 5, line 17) and the products contain very high proportions of free TDA (1 to 2% by weight).
The preparation by a two-stage process of low viscosity polyethers based on o-TDA within the OH number range of from 300 to 500 is described in U.S. Pat. No. 4,562,290. In this process, 1 to 3 moles of ethylene oxide per mole of o-TDA are added in the first stage at 125.degree. C. and, after the addition of potassium hydroxide as catalyst, propylene oxide is then added at a temperature of at least 140.degree. C.
The polyethers based on TDA prepared as described in U.S. Pat. No. 4,209,609 have only limited applications. Such polyethers cannot be used for the manufacture of rigid foam boards with thicknesses greater than 10 cm, although such boards are increasingly required by building regulations in many countries and can be produced on conventional laminators. Although, as already mentioned, such polyethers give rise to foams with low coefficients of thermal conductivity, high flame resistance, and good toughness combined with good flow properties, the products have a strong tendency to form cracks and to discolor at the core, especially when made in thick layers.
It has now surprisingly been found that TDA and MDA polyethers having excellent foaming properties can be used to prepare products in thick layers without the above-mentioned disadvantages of crack formation and core discoloration and with desirable improvements in properties such as flame resistance, toughness, and, especially, coefficients of thermal conductivity even lower than those obtained by known methods. Such improvements are achieved if the TDA is a 2,4- and/or 2,6tolylenediamine or a mixture thereof with o-tolylenediamine and the MDA consists of diphenylmethane di- and/or polyamines and/or methyldiphenylmethane di- and/or polyamines as described in EP 024,665, 046,556, and 046,917, and if the entire alkoxylation with ethylene oxide followed by propylene oxide is carried out in the presence of special amine catalysts as described below.